1. Field of the Invention
The present invention generally relates to apparatuses for mounting a superconducting element and, more particularly, to an apparatus for mounting a superconducting element needed to be cooled.
Recently, a large number of superconducting integrated circuits utilizing Nb Josephson elements has been reported. The circuits utilizing the Josephson-junction elements offer high-speed operation and low power dissipation, and hence make it possible to achieve high-speed processors. In operation, the superconducting element must be kept at a very low temperature (269.degree. C. below zero for a Nb element). Hence, the apparatus for holding the superconducting element at a very low temperature is a very important factor.
2. Description of the Prior Art
Normally, liquid helium is used as a coolant to operate the Nb Josephson-junction element. More particularly, a superconducting circuit chip is placed in a Dewar vessel accommodating liquid helium, and is electrically connected to a device placed in the room-temperature atmosphere by means of a coaxial cable. This structure needs the coaxial cable to be 1 m long at least. In this case, the propagation delay time is estimated to be approximately 10 ns. Such a delay time does not operate the superconducting circuit at high speed (for example, 1 ns or less).
With the above in mind, the following apparatus for mounting a superconducting element has been proposed (Japanese Patent Application No. 63-276023).
FIG. 1 shows an apparatus 1 for mounting a superconducting element proposed in the above Japanese patent application. The mounting apparatus 1 accommodates a circuit board 3 on which an integrated circuit chip 2 to be cooled, such as a Nb Josephson-junction element is mounted. The circuit board 3 is placed in a coolant of liquid helium in an inner housing of a Dewar vessel (cooling chamber) 7, which has an outer housing outside of the inner housing. An electric-signal cable 9 penetrates through a vacuum adiabatic layer 8 formed between the inner housing and the outer housing, and electrically connects the cooled integrated circuit chip 2 and a room-temperature-operation circuit chip 4 mounted on a circuit board 5. The above structure makes it possible to arrange the chip 2 and the chip 4 close to each other and hence reduces the length of the electric-signal cable 9 between the cooled integrated circuit chip 2 and the room-temperature-operation chip 4 to one-tenth or less. As a result, high-speed synchronizing operation on the chips 2 and 4 can be achieved.
A cooling head 11 connected to a refrigerating machine 10 is provided in the upper portion of the Dewar vessel 7, and reliquidizes helium evaporated by heat from the cable 9 and heat generated by the chip 2. In this manner, the chip 2 is continuously cooled by the liquid helium 6.
However, the apparatus shown in FIG. 1 has a disadvantage in that the Dewar vessel 7 has a single cooling area, and hence there is no degree of freedom in the arrangement of the refrigerating machine 10 and the cooling head 11. The refrigerating machine 10 and the cooling head 11 are necessarily disposed in the upper position of the Dewar vessel 7 taking into account the following.
In order to establish the stable operation of the cooled integrated circuit chip 2, it is necessary to place the chip 2 in the liquid helium 6 to keep the operation temperature constant. The liquid helium 6 is evaporated due to heat generated by the chip 2 and heat from the cable 9. The cooling head 11 cools the evaporated helium and reliquidizes it, so that the quantity of liquid helium 6 is kept constant. In this case, in order to reliquidize the evaporated helium by the cooling head 11, it is necessary to keep the temperature of the cooling head 11 lower than the temperature of the liquid helium 6. This is because the temperature difference functions to take heat from the evaporated helium. Of course, the liquid stays in the lower portion of the cooling chamber 7. Hence, the refrigerating machine 10 and the cooling head 11 must be disposed in the upper portion of the cooling chamber 7.
It is known that generally, the efficiency of the refrigerating machine 10 for cooling the chip 2 is not good. The refrigerating machine 10 tends to have a larger volume and a larger weight (see S. Kotani, et al., "A Sub-ns-Clock Cryogenic System for Josephson Computers", IEEE Transactions on Applied Superconductivity, Vol. 1, No. 4 Dec., 1991). Hence, it is necessary to arrange the large-volume, heavy refrigerating machine in the upper portion of the cooling chamber 7. This needs a large and strong supporting mechanism, which leads to an increase in size of the overall mounting apparatus.